When I used my old Canon EOS 600 many moons ago I used an accessory thing to use the old style cable release as the EOS didn't have the screw thread in the shutter button.
The thing I bought was a velcro strap with a metal boss on it that took the old cable release, and they could be used either as normal shutter press or set to a lock position.
It just strapped around the camera over the shutter button, and the velcro meant it was universal and not just for the EOS, and the cable release pushed the camera shutter button.
Wow!
Google is your friend.
KAISER Cable Release Adapter for 13.99atJacobs Digital Photography & Video
This is the exact thing, and it takes this type of cable release.
CALUMET 12" Standard Cable Release at cheap prices | PC World
Those are UK sites, but the products are surely available anywhere.